It is known that in a rotorcraft, the incident flow conditions onto a rotor are constantly changing even in a steady-state flight regime. These changes result from a superposition of the forward speed of the rotorcraft and the rotation speed of the rotor.
This superposition results substantially in two extreme incident-flow situations, specifically at the advancing blade and the retreating blade. At the advancing blade, the rotational component is superimposed positively on the translational component, and results in an incident flow with a highly transonic expression at the blade tip. In this case a maximally thin profile having a small curvature is desirable in order to prevent severe supersonic shocks. At the retreating blade, the translational component resulting from the forward flight speed is subtracted from the rotational component. This leads to a sharp reduction in the local incident flow speed, to the point that in the inner blade region, the flow is incident on the profile from behind. In combination with the high blade angle present there, the low incident flow speed results in considerable flow detachment, which negatively affects the control rod loads of the rotor. In this region, a thicker profile or a profile having a large curvature in the nose region would be advantageous.
A profile whose geometry is modifiable is proposed, and which takes into account these extreme flow regions and adjusts itself adaptively to the particular conditions that exist.
The great variation in aerodynamic conditions at the rotor in the course of a revolution results overall in vibrations of the rotor blades at a multiple of the rotation frequency.
A further disadvantageous aerodynamic effect exists during approach by the aircraft at certain sink angles. An interaction can occur in this context between the blade eddy and the blades, leading to a pulsing noise phenomenon. This phenomenon can be mitigated by deforming the rotor downwash at a suitable azimuthal position, or modifying the spatial location of the blade, in such a way that the above-described interaction of the eddy and blade does not occur. This relative change in the spacing of the blade and wake is achieved by a brief modification in lift.
In principle, two possibilities exist for modifying blade lift:                1. Directly: As in the case of a fixed-wing aircraft's wing, lift is modified by a downward flap deflection (downward flap deflection=>increased lift); or        2. Indirectly: The flap deflection generates, by way of a modified moment coefficient, a torsional moment that adjusts the angle of attack of the torsionally flexible blade by means of the induced torsional deformation (upward flap deflection=>torsional deformation=>change in angle of attack=>increased lift). This effect is also referred to in the literature as a “servo effect.”        
The second possibility is used more often at present, since it appears more effective in the context of available positioning drive performance levels.
A rotor blade utilizing the servo effect and having a movably mounted flap is known from DE 101 16 479 A1. The flap is movably attached to the rotor blade with the aid of rolling bearings. Control is applied to the flap via a piezoactuator that is arranged at a spacing in a front (as viewed in the profile depth direction) region of the rotor blade. The piezoactuator generates positioning forces, and transfers them via tension elements to the rotor blade flap.
In practical use, rotor blades having discrete flap bearings are subject to elevated wear because of their high frequency and as a result of dust, dirt, and water in the environment. A shortened service life and increased maintenance requirement are the results.
DE 103 34 267 A1 therefore proposes a rotor blade having a rotor blade flap embodied in bearingless and hingeless fashion. In the disclosed rotor blade, piezoelectric actuators are mounted into the rigid covering skins of the blade profile or directly below the inherently rigid covering skins, or on the rigid covering skins, so that one of the two piezoelectric actuators on the upper-side covering skin or lower-side covering skin of the blade profile can selectably be actuated, and thus cause a displacement of the respective covering skin relative to the other covering skin; this shortens or lengthens the upper covering skin relative to the lower covering skin. The relative shortening of one covering skin with respect to the other causes the rigid rotor blade flap attached to the covering skins to be deflected, and to be moved upward or downward.
The known rotor blades of the kind previously recited have the disadvantage that an upwardly deflected flap causes the generation of lift, which results in a rise in drag as compared with a neutral or downwardly deflected profile.